1. Technical Field
The present invention relates to a robot controller, a robot system, and a robot control method.
2. Related Art
Operations using robots such as manipulators include operations with various constraints, for example, in contact with objects. In these cases, force control is often required in addition to location control. For example, in the cases of tracing the surface of an object, fitting one object into another object, and grasping a soft object so as to not break it, and the like, movement in response to the reaction force from the object is necessary in addition to simple location control.
Representative methods of force control in robots include a method called impedance control. Impedance control is a control method of moving a robot, regardless of its real mass, viscosity property, and elasticity property, as if it had those values suitable for an operation. This is a control method of solving an equation of motion based on force information obtained from a force sensor attached to the robot and moving the robot according to the solution. By appropriately setting the equation of motion, a robot such as a manipulator can be moved as if it had predetermined mass, viscosity, and elasticity.
Note that, in impedance control, in order to allow the robot to behave as if it had desired properties (mass, viscosity property, elasticity property), it is necessary to solve an ordinary differential equation using coefficient parameters corresponding to the properties (an equation of motion as a second-order linear differential equation). Various methods of solving the differential equation have been known, and the Runge-Kutta method, the Newton method, or the like is used.
As a related art with respect to impedance control and force control, a technology disclosed in JP-A-10-128685 is known.
Since the above described differential equation in impedance control is linear, the impedance control has a symmetric characteristic with respect to the direction (orientation) of force (external force).
However, when a human performs an operation, sometimes he or she controls force so that displacement may be asymmetric with respect to the external force direction. For example, when an object is inserted into a device having some pull-in mechanism, force is applied against the repulsive force from the device, and, once the pull-in mechanism is activated and the force is changed to pull-in force, the human moves according to the force, i.e., has a “soft” characteristic. Therefore, when the robot performs an operation like a human, asymmetric impedance control is necessary.
Here, in JP-A-10-128685, a method of providing non-linearity by using a function that is non-linear with respect to virtual displacement for the compliance term (elasticity term) of the equation of motion in the force control is disclosed. There is no explicit description in the specification of JP-A-10-128685, however, it is conceivable that the non-linear compliance term includes an asymmetric compliance term, and thereby, it is also conceivable that asymmetric impedance control may be realized.
However, the method provides the asymmetry with respect to the location (displacement), but does not provide asymmetry of the displacement (correction value) with respect to the force direction as described above. That is, according to the method disclosed in JP-A-10-128685, the impedance control providing asymmetric displacement with respect to the force direction may be realized.